Active-matrix liquid crystal display devices typically include an active matrix substrate in which thin-film transistors (TFTs) that function as switching elements are formed for each pixel, an opposite substrate in which color filters or the like are formed, and a liquid crystal layer sandwiched between the active matrix substrate and the opposite substrate. An electric field is applied to the liquid crystal layer by creating a difference in electric potential between a common electrode and pixel electrodes that are electrically connected to the TFTs. This electric field changes the alignment state of the liquid crystal molecules in the liquid crystal layer, thereby making it possible control the transmittance of the pixels in order to display images.
Various display modes have been designed for and are used in active-matrix liquid crystal display devices according to the intended use case. Examples of such display modes include twisted nematic (TN) mode, vertical alignment (VA) mode, in-plane-switching (IPS) mode, and fringe field switching (FFS) mode.
In these types of liquid crystal display devices, the active matrix substrate sometimes includes two transparent conductive layers with an inorganic insulating layer formed therebetween. Here, this electrode structure that includes two transparent conductive layers with an inorganic insulating layer sandwiched therebetween will be referred to as a “bilayer electrode structure” for simplicity.
In a typical FFS mode device, for example, the lower transparent conductive layer functions as the common electrode, and the upper transparent conductive layer has a plurality of slits formed therein and functions as the pixel electrodes.
The applicant conducted research and development on liquid crystal display devices that utilize such a bilayer electrode structure and exhibit auxiliary capacitance. More specifically, the applicant researched configurations in which the lower transparent conductive layer functions as an auxiliary capacitance counter electrode (to which a common voltage or an auxiliary capacitance counter voltage is supplied) and the upper transparent conductive layer functions as the pixel electrodes. This liquid crystal display device is a VA mode device, for example, but the same configuration may be applied to devices of other display modes as well.
Patent Documents 1 to 3, for example, disclose examples of active matrix substrate configurations that have this bilayer electrode structure. In configurations in which the upper transparent conductive layer functions as the pixel electrodes, the upper transparent conductive layer must be electrically connected to the drain electrodes of the TFTs, which are positioned beneath the lower transparent conductive layer.
In the active matrix substrate disclosed in Patent Document 1, for example, an interlayer insulating layer that covers the TFTs and an inorganic insulating layer that is sandwiched between the two transparent conductive layers are both etched to form contact holes therein, and the drain electrodes of the TFTs contact the pixel electrodes inside these contact holes.
Moreover, in the active matrix substrate disclosed in Patent Document 2, an opening formed in an interlayer insulating layer and an opening formed in an inorganic insulating layer are arranged such that an intersecting cross pattern is formed when the substrate is viewed from above. The pixel electrodes are arranged contacting the drain electrodes at the intersections between these two types of openings, where the drain electrodes are exposed.
Meanwhile, Patent Document 3 discloses a configuration in which the drain electrodes are connected to the pixel electrodes via transparent connection layers (relay layers) formed inside contact holes. Here, the connection layers can be formed from the same transparent conductive film as the lower transparent conductive layer.